This invention relates to spectroscopic detection.
In one application, spectroscopic monitoring devices monitor emissions from industrial smokestacks for the presence of toxic species that pose a health threat to humans. In particular, these devices monitor the level of metals, e.g., mercury, lead, arsenic, beryllium, cadmium, and chromium, emitted by thermal processes such as waste incineration, fossil fuel burning power plants, chemical manufacturing, and metals refining.
Spectroscopic monitors are also used in environmental and industrial processing and testing applications to tests samples, e.g., water and gas, for trace amounts of elements or compounds. For example, spectroscopic monitors are used in combination with plasma systems such as inductively coupled plasma (ICP) and microwave-sustained plasma to detect trace amounts of metals by atomic emission spectroscopy.
In an aspect, the invention features a spectroscopic device including dispersing optics for dispersing electromagnetic radiation, a detector for measuring electromagnetic radiation, a transmissive rotating blade for causing alternate frequency ranges of the electromagnetic radiation to move onto and off of the detector, and detector electronics for monitoring an electromagnetic radiation signal at a first frequency range while correcting a background contribution using an electromagnetic radiation signal at a second frequency range.
In another aspect, the invention features a spectroscopic device including dispersing optics for dispersing electromagnetic radiation, a detector for measuring electromagnetic radiation, a reflective rotating blade before the dispersing optics for causing electromagnetic radiation to move onto and off of the detector, an attenuator between the reflective rotating blade and the detector for controlling the intensity of the electromagnetic radiation directed onto the detector, and detector electronics for monitoring an electromagnetic radiation signal at a first frequency range while correcting a background contribution using an electromagnetic radiation at a second frequency range.
In still another aspect, the invention features a spectroscopic device including dispersing optics for dispersing electromagnetic radiation, a detector for measuring the electromagnetic radiation, a movable transmissive element having different portions for causing, by a difference in the refraction between the different portions, alternate frequency ranges of the electromagnetic radiation incident on the different portions to move on to and off of the detector, and detector electronics for monitoring an electromagnetic radiation signal at a first frequency range while correcting a background contribution using an optical signal at a second frequency range.
Embodiments may also include one or more of the following features. The transmissive rotating blade, reflective rotating blade, or movable transmissive element includes a first portion and a second portion having different thicknesses. The first and the second portions have the same or different refractive indices. The transmissive rotating blade, reflective rotating blade, or movable transmissive element includes material free portions which pass electromagnetic radiation without refraction. At least a portion of the transmissive rotating blade is formed of quartz. The transmissive rotating blade includes multiple stacked quartz sections of different shapes to create regions of the blade having different thicknesses. The detector electronics include a lock-in amplifier. The detector electronics further include a sensor configured to measure the frequency at which the transmissive rotating blade, reflective rotating blade, or movable transmissive element moves the alternate frequency ranges onto and off of the detector. The lock-in amplifier is configured to receive electric signals from the sensor and the detector. The spectroscopic device further includes a transfer line configured to transmit the electromagnetic radiation from a radiation source to the dispersing optics. The transmissive rotating blade, reflective rotating blade, or movable transmissive element rotates at a frequency sufficient to reduce the background contribution. The electromagnetic radiation signal at the first frequency range includes contributions from contaminant emission and background emission. The electromagnetic radiation signal at the second frequency range includes contributions from background emission. The spectroscopic device further includes an emission spectrum generator. The spectroscopic device is arranged to receive electromagnetic radiation from combustion effluent. The spectroscopic device is arranged to detect electromagnetic radiation emitted from metals.
The invention also features the modulators themselves and methods of spectroscopic detection and monitoring as discussed below.
Embodiments may include one or more of the following advantages. In embodiments, the invention features monitoring modulated light signals to detect low levels of contaminants and other compounds in the presence of background interference. The spectroscopic monitor can extract low-level signals of contaminants from high-level background signals by modulating the light signals and monitoring them with frequency sensitive detection. The spectroscopic monitor can cancel out changes and drift in the background signal by continuously measuring the low-level signals relative to high-level background signal. Canceling drift or changes in the background signal increases the signal-to-noise ratio of the monitoring signal and reduces the uncertainty associated with quantifying the absolute amount of contaminant present in a sample. In embodiments, the spectroscopic monitor increases the detection limit significantly in situations where there is interfering background emission and can measure the amount of contaminants at detection limits lower than 1 xcexcg/m3. Detecting elements or compounds at low levels is important in a number of areas such as forensic sciences, environmental sciences, food sciences, pollution monitoring, and the chemical and pharmaceutical industries. In particular, the spectroscopic monitor can be used for environmental compliance monitoring, to test the purity of food, medicines, and new bio-engineered products by quantifying the level of contaminants.
The dispersing optics of the spectroscopic monitor are substantially stationary and are less prone to misalignment. As a result, the spectroscopic monitor is robust and can be used in a wide variety of harsh environments, e.g., in monitoring emissions from industrial smokestacks.
Other features, aspects and advantages follow.